static Pair<JTabbedPane, Int> enclosingTab(Component _c) {
  ret swing(() -> {
    Component c = _c;
    while (c != null) {
      Container p = c.getParent();
      if (p cast JTabbedPane)
        ret pair(p, p.indexOfComponent(c));
      c = c.getParent();
    }
    null;
  });
}